Single side band (SSB) modulators are used in many communication ECM systems. Most of these modern systems employ solid state modulators which have been designed using both active and passive components in a variety of circuit configurations. These configurations include single balanced structures and double balanced structures. Unwanted side band suppression and carrier rejection are important performance issues for such structures.
One way of creating a single side band signal is the phase cancellation method. A low frequency (LF) signal, typically an information-bearing signal, is split into two identical but 90° phase shifted signals. The two low frequency signals are said to be in quadrature. A radio frequency carrier signal is provided by local oscillator (LO), and is also split into separate signals, so that the separate signals have a 90° phase shift relative to each other. One radio frequency carrier signal and one low frequency (LF) signal are combined in each of two balanced mixers or modulators, and the output signals of the mixers are summed. The system acts to suppress the carrier signal, and to provide an increased amplitude single side band signal.
FIG. 1 is a block diagram of a typical single sideband modulator 100 known to the prior art. In such a single side band system, it is well known in the art that when a carrier signal and modulating signal are mixed in a double balanced mixer or a modulator, the output thereof is generally the sum or difference of the signals. The carrier signal or local oscillator (LO) signal is provided at the local oscillator port 105, or LO 105. The modulating signal or LF signal is provided in this figure at two points. A first LF signal 110 and a second LF signal 115 are identical modulating signals that have equal amplitudes, but are 90 degrees out of phase. An LO signal applied at LO 105 is fed into the 90 degree hybrid 120. This will spilt the LO signal into two halves. A first split LO signal 125 and a second split LO signal 130 differing in phase by 90 degrees will be outputted by the 90 degree hybrid 120. The first LO split signal 125 is fed into a connected first mixer 135. The second LO split signal 130 is fed into a connected second mixer 140. The first LF signal 110 is fed to the first mixer 135, and the second LF signal 115 is fed to the second mixer 140. A first RF signal 145 is output from the first mixer 135 to the in-phase power combiner 160. A second RF signal 150 is output from a second mixer 140 to the in-phase power combiner 160. Within the in-phase power combiner 160, the two signals, 145 and 150, are summed to provide a single radio frequency side band signal without a carrier signal.
To obtain high performance, the first and second mixers 135 and 140 are typically double balanced mixers, or DBMs. A prior art DBM 200 is shown in FIG. 2. Such a DBM 200 consists of a local oscillator (LO) port 202, a LO transformer 204 for providing balanced output to quad ring 208, a radio frequency (RF) port 206, an RF transformer 218 for providing balanced signals from the quad ring 208 to the RF port 206. The balanced signals drive a schottky diode quad ring 208. The schottky diode ring 208 is a combination of four (quad) schottky diodes arranged in a ring configuration. The modulating [LF] signal of this circuit is at the intermediate frequency or IF port 210. In the SSB modulator 100 (FIG. 1), it is the signal at RF port 206 that is fed into the in-phase power combiner 160. Thus we see that in a prior art SSB modulator 100 there will be at least two transformers for each of the mixers, making a total of four or more transformers due to the mixers.
The circuit diagram of a prior art in-phase power divider/combiner 300 is shown in FIG. 3. Note that such a device can be either a combiner or a divider, but in SSB modulator 100, it is used as a combiner. The two modulated RF signals 145 and 150 are applied to ports 305 and 310. A resistor 315 connects the two ports 305 and 310. In parallel to the resistor 315 is the first transformer 320. The first transformer is connected to port 305 on one winding and port 310 at the other winding. A capacitor 325 is connected to the first transformer 320, which is in turn connected to a second transformer 330. The output port 335 is connected to the second transformer 330.
Diagrams of embodiments of a prior art 90 degree hybrid are shown in FIG. 4. FIG. 4A shows a circuit diagram of a narrow band 90 degree hybrid. FIG. 4B shows the amplitude response versus the frequency of such a 90 degree hybrid. FIG. 4C shows a block diagram of a wide band 90 degree hybrid. FIG. 4D shows the amplitude response versus the frequency for such a wide band 90 degree hybrid.
Thus, the prior art implementation of the SSB modulator will have even more transformers, and other components. It is desirable for reasons of cost-efficiency, reliability, and performance to reduce the number of components in high frequency applications of an SSB modulator.